1. Field of the Invention
This invention relates to optical devices which include refractive index modulation, e.g. reflection gratings.
2. Related Art
Reflection gratings are often implemented as waveguides which have a path region and/or a confining region with a modulated refractive index. The waveguiding structure is often in the form of a fibre. The modulation preferably takes the form of alternate regions of higher and lower refractive index. When radiation traverses the modulation, it is selectively reflected. The period of the refractive index modulation is usually equal to the wavelength to be reflected or to a multiple or sub-multiple of said wavelength. Thus periods in the range 250 to 600 nm preferentially reflect selected wavelengths within the range 800-1650 nm.
Reflection gratings have many applications in optical signalling. For example, a reflection grating can be associated with a fibre laser in order to narrow the lasing bandwidth. When the refractive index bands are not perpendicular to the fibre axis, the grating can be used for the selective removal of unwanted wavelengths. In addition to reflection gratings, refractive index modulation has other applications, e.g. to achieve phase matching in waveguides, to control spot size and/or shape in waveguides and for storing information.
Refractive index modulation is conveniently produced by an optical process in which a photosensitive glass is exposed to radiation which causes an adequate change in its refractive index. The radiation has higher and lower intensities corresponding to the intended pattern of modulation of the refractive index of the glass. In many commonly used embodiments, the mutual interference of two beams of radiation produces the variation of intensity appropriate for reflection gratings. In the case of information storage, the pattern of radiation relates to the data to be stored.
Silica/germania glasses are widely used in optical telecommunications and it has been noticed that these glasses have an optical absorption band extending approximately over the wavelength range 225-275 nm and exposure to radiation within this band increases the refractive index of the silica/germania composition. The peak of the band occurs at a wavelength which is close to 240 nm. It has, therefore, been proposed to produce refractive index modulation, e.g. to make reflection gratings, by exposing silica/germania glass compositions to radiation within the wavelength band 225-275 nm. Radiation close to 240 nm is particularly suitable. High powers of radiation, e.g. above 1 mW continuous, are needed to produce adequate changes in the refractive index and writing times of a few minutes to a few hours are appropriate.
WO86/01303 describes the writing of phase gratings in optical fibres or waveguides by the application of intense beams of ultraviolet light. It is stated that the grating produced in the core of a wave guide and that the core is preferably a germanium-doped silica or glass filament.